Several patents and publications are cited in this description in order to more fully describe the state of the art to which this invention pertains. The entire disclosure of each of these patents and publications is incorporated by reference herein.
The use of bitumen in the manufacture of materials for highway and industrial applications has been known for a long time. Bitumen is the main hydrocarbon binder used in the fields of road construction and civil engineering. To be useful as a binder in these different applications, the bitumen must have certain mechanical properties, and in particular certain elastic or cohesive properties. The mechanical properties of the bituminous compositions are measured by standardized tests, such as the softening point, the penetration, and the rheological characteristics in defined traction. Asphalts are performance graded (PG) by a set of specifications developed by the U.S. federal government (Strategic Highway Research Program or SHRP). For example, PG58-34 asphalt provides good rut resistance at 58° C. and good cold cracking resistance at −34° C., as determined by standards of the American Association of State Highway Transportation Officials (AASHTO).
In general, the conventional bitumens do not have all of the qualities required for use in road construction according to current standards. Therefore, various polymers have been added to the conventional bitumens to modify their mechanical properties and to form bitumen-polymer compositions having improved mechanical qualities compared with those of the bitumens alone.
Asphalt sold for paving may be modified with polymers to improve rut resistance, fatigue resistance, and cracking resistance. In addition, polymer modification may decrease the stripping from aggregate that results from increases in asphalt elasticity and stiffness. Further, the addition of polymer to asphalt provides higher temperature rut resistance and also improves fatigue resistance. Good low-temperature properties, such as penetration index, are to a large extent dependent on the specific asphalt composition (e.g., flux oil content); however, the polymer type does influence low temperature performance.
The asphalt industry classifies polymers for asphalt modification as elastomers or plastomers. Generally, elastomeric polymers improve low-temperature performance and plastomeric polymers cause deterioration in low-temperature performance. The word “plastomer” indicates a lack of elastomeric properties. Plastomers are sometimes used to modify asphalt because they can increase stiffness and viscosity, which improves rut resistance. They are generally considered inferior to elastomers, however, due to lack of significant improvements in fatigue resistance, creep resistance, cold crack resistance, etc. Styrene/butadiene/styrene block copolymers (SBS) are considered to be elastomers, as are ethylene/butyl acrylate/glycidyl methacrylate terpolymer (EnBAGMA) and ethylene/vinyl ester/glycidyl methacrylate terpolymer (EEGMA) resins, both available from E. I. du Pont de Nemours and Company of Wilmington, Del., USA (“DuPont”) under the trademark Elvaloy® RET. Polyethylene (PE) and ethylene vinyl acetate (EVA) resins are considered plastomers. Polyethylene does not form a stable solution with asphalt; therefore, polyethylene-modified asphalt must be continuously stirred to prevent separation. Further, polyethylene-modified asphalt must be prepared at the mix plant and cannot be shipped due to separation. Polyethylene therefore acts as filler and does not meaningfully increase the softening point of asphalt.
Among the polymers typically added to bitumens, random or block copolymers of an aromatic monovinyl hydrocarbon and a conjugated diene and in particular copolymers of styrene and butadiene (SBS) or of styrene and isoprene are particularly effective. They dissolve very easily in most bitumens and confer excellent mechanical and dynamic properties, in particular very good viscoelastic properties. U.S. Pat. No. 6,087,420 describes a method for producing bitumen/polymer compositions comprising at least one styrene-butadiene copolymer.
The use of other polymers as additives to asphalt (bitumen) is also known in the art. See, for example, U.S. Pat. Nos. 4,650,820 and 4,451,598, which describe bitumen mixed with terpolymers derived from ethylene, an alkyl acrylate and maleic anhydride.
Also see for example U.S. Pat. Nos. 5,306,750; 6,117,926; and 6,743,838; and U.S. Patent Application Publication No. 2007/0027261, which describe bitumen mixed and reacted with epoxy-functionalized, particularly glycidyl-containing, ethylene terpolymers and, preferably (as described in U.S. Pat. No. 6,117,926), with a catalyst to accelerate the rate of reaction and lower cost of the modified system. Consistently with these descriptions, DuPont Elvaloy® RET resins (ENBAGMA and EEGMA) are excellent modifiers for asphalt and improve asphalt performance at concentrations as low as 1 to 2 weight %.
Without wising to be held to hypothesis, it is believed that the improvement in asphalt properties with addition of Elvaloy® RET at such low concentrations is due to a chemical reaction between the Elvaloy® RET and the functionalized polar fraction of asphalt (“asphaltenes”).
U.S. Pat. No. 5,331,028 describes blends of asphalt with a combination of glycidyl-containing ethylene copolymer and a styrene-conjugated diene block copolymer.
U.S. Pat. No. 9,028,602 describes a bituminous composition comprising a bitumen in an amount ranging from 20 to 90 weight %, a carboxylic additive in an amount of from 0.25 to 5 weight %, and sulfur in an amount of 5 to 75 weight %, all percentages based on the weight of bitumen, carboxylic additive and sulfur, wherein the carboxylic additive is selected from carboxylic acids, carboxylic esters and carboxylic anhydrides.
Combining asphalt with elastomers such as EnBAGMA and EEGMA requires significant mixing at elevated temperatures to achieve the benefits of their addition. EnBAGMA and EEGMA are presented in pellet form and are added with stirring to hot asphalt, so that the pellets soften and melt.
The reaction between the polymer and the bitumen occurs with heat alone; however, acids such as superphosphoric acid (SPA) or polyphosphoric acid (PPA) are sometimes added to reduce the reaction time. Addition of acid can be viewed negatively in some cases, however. For example, in some jurisdictions the use of acid to accelerate mixing of polymers with asphalt is discouraged or even prohibited. In addition, some PMA producers believe that acid degrades the asphalt's properties or that acid is incompatible with amine based materials, such as the ones used as anti-stripping agents. Common anti-stripping agents include polyamines such as tetraethylenepentamine (TEPA) and bishexamethylenetriamine (BHMT); fatty amines; and amidoamines derived from fatty acids which in turn are derived from natural oils such as coconut oil and tall oil.
The reaction between the Elvaloy® RET and the asphaltenes does occur with heat alone, although the rate is lower (about 6 to 24 hours without acid and about 3 to 6 hours with acid). In addition, asphalt modified with Elvaloy® RET in the absence of SPA is less elastic than asphalt modified with Elvaloy® RET in the presence of SPA, as evidenced by a higher phase angle and lower elastic recovery. Some PMA producers prefer acid catalysis and some prefer to use heat alone. Driving the modification reaction kinetics with heat alone does eliminate the problem with amine-based anti-strippping agents, however.
It is also known that the stability of the bitumen/polymer compositions can be improved by chemical coupling of the polymer with the bitumen, this improvement moreover making it possible to extend the field of use of the bitumen-polymer compositions. The chemical coupling of the polymer with the bitumen may be accomplished, for example, by cross-linking the polymer using a cross-linking agent, such as a sulfur-donor compound.
The cross-linking of the bitumen/polymer compositions confers upon them very good properties in terms of storage stability, cohesion, elongation capacity, and resistance to aging.
For example, bitumen-polymer compositions for which a random or block copolymer of styrene and a conjugated diene such as butadiene or isoprene is coupled with the bitumen can be prepared using the processes described in the publications FR-A-2376188, FR-A-2429241, FR-A-2528439 and EP-A-0360656. In these processes, the source of sulfur consists of chemically non-bound sulfur (FR-A-2376188 and FR-A-2429241), in a polysulfide (FR-A-2528439) or in a sulfur-donor vulcanization accelerator used alone or in combination with chemically non-bound sulfur and/or a polysulfide or a non-sulfur-donor vulcanization accelerator (EP-A-0360656).
In addition to the sulfur-based crosslinking agents, sulfur in the asphalt or bitumen compositions can arise from two natural sources. First, asphalt is produced in the refinery process by removing the higher value components such as gasoline, oils and light ends from crude oil. Contaminants such as sulfur and sulfur components are concentrated in the refinery bottoms from which the bitumen is derived. Consequently, depending on the source of the crude oil, the resulting asphalt may include crudes and refinery bottoms with high levels of sulfur and sulfur components. Second, the viscosity of asphalt is high. Therefore, it is typically stored at temperatures over 140° C., conditions that can promote further thermal cracking of sulfur-containing compounds.
The presence of sulfur, however, whether as a result of the crude oil composition, the asphalt storage conditions, or the use of sulfur-donor cross-linking agents, leads to significant emission of hydrogen sulfide (H2S) during PMA production and storage. Hydrogen sulfide is a colorless and toxic gas, having a characteristic odor at a very low concentration. Asphalts may exhibit high levels of H2S, often exceeding 20,000 ppm. In PMA production units, the concentration of H2S released during the manufacture of a cross-linked bitumen-polymer composition is particularly significant. The release of H2S is much greater for the cross-linked bitumen-polymer compositions than for bitumen bases devoid of cross-linking agent. The use of acid to accelerate the mixing of the polymer modifiers with the asphalt can also significantly increase the release of H2S. For reasons of safety and because of environmental constraints, the reduction or even the elimination of H2S emissions during the production of PMA constitutes a crucial industrial challenge.
For at least the foregoing reasons, it remains desirable to prepare polymer-modified asphalt compositions without using acid to accelerate the blending process.